A polymer electrolyte membrane fuel cell (PEMFC) generates electricity by electrochemical reaction between hydrogen as fuel gas and oxygen (or air) as oxidant gas, which are reactant gases.
The PEMFC has high efficiency, high current density, high power density, short start-up time, and rapid response to a load change, compared to other types of fuel cells. Accordingly, the PEMFC has been used in various applications, such as a power source for zero-emission vehicles, an independent power plant, and a military power source.
In general, a fuel cell has a stack structure in which cells are stacked and assembled in order to satisfy a required power level. Accordingly, a fuel cell mounted in a vehicle also has a stack structure in which several hundreds of cells are stacked in order to satisfy the required high power level.
A membrane-electrode assembly (MEA) is positioned in the center of each unit cell of the fuel cell stack. The MEA includes a solid polymer electrolyte membrane through which hydrogen cations (protons) are transported, and a catalyst electrode configured by applying catalysts on both surfaces of the electrolyte membrane. That is, the catalyst electrode includes an anode (hydrogen electrode) and a cathode (air electrode).
In addition, a gas diffusion layer (GDL), a gasket for preventing a gas leak, etc. are stacked outside the MEA, namely at outer portions where the anode and the cathode are located. A bipolar plate has flow fields, through which reactant gases, coolant, and water produced by reaction flow, and is bonded to an outer side of the GDL.
According to the conventional art as described above, an oxidation reaction of hydrogen as a fuel occurs in the anode of the fuel cell stack to generate hydrogen ions and electrons. The generated hydrogen ions and electrons are transmitted to the cathode through the electrolyte membrane and the bipolar plate, respectively.
Thus, electrical energy is produced by the flow of electrons, and water and heat are produced by the electrochemical reaction, in which the hydrogen ions and electrons transmitted from the anode and oxygen in the air participate, in the cathode.
The bipolar plate divides the unit cells in the fuel cell stack, and at the same time, serves as a current path (a path for transferring generated electricity) between the unit cells. The flow fields formed in the bipolar plate serve as a path for transferring reactant gases to the GDL, a path for the pass of coolant, and a path for discharging water, which is produced by the electrochemical reaction and is discharged through the GDL, to the outside.
Such a bipolar plate includes a graphite bipolar plate made of a graphite material, and a metallic bipolar plate made of a metal material such as stainless steel. A study to replace the graphite bipolar plate with the metallic bipolar plate is actively ongoing in consideration of processability and mass production.
However, it is difficult for the metallic bipolar plate manufactured by press working to realize a complex shape. For this, the metallic bipolar plate uses a thin plate material, and thus, it is possible to reduce the thickness and weight of the bipolar plate and the volume of the unit cell.
In general, after bipolar plates are manufactured by forming relief/intaglio patterns in a metal plate material through press working in a mold, two bipolar plates are coupled to each other. Accordingly, coolant flows in a channel space defined by contact of the bipolar plates, and GDLs are disposed at both sides of the bipolar plates so that hydrogen and oxygen flow in respective channel spaces defined between the GDLs and the bipolar plates so as to transfer reactant gases.
FIG. 1 is a top view illustrating a typical metallic bipolar plate structure for a fuel cell. Referring to FIG. 1, a bipolar plate 10 generally has a rectangular shape. The bipolar plate 10 has a reaction region 11 which has flow fields for air, hydrogen, and coolant. Opposite end portions of the reaction region 11 have inlet manifold holes 12, 14, and 16 and exit manifold holes 13, 15, and 17 through which air, hydrogen, and coolant enter and exit, respectively.
Humidified air and hydrogen are supplied as reactant gases from an external source of a stack through the air and hydrogen inlet manifold holes 12 and 14 to operate the fuel cell. Gas-phase or liquid-phase water produced in the fuel cell in addition to the supplied reactant gases is discharged through the air and hydrogen outlet manifold holes 13 and 15 to the outside of the stack.
That is, the reactant gases and the water produced in the cell are discharged through the air outlet manifold hole 13, and the reactant gases and the water, which is produced in a cathode and then permeates an electrolyte membrane to be transmitted to an anode, are discharged through the hydrogen outlet manifold hole 15.
In each bipolar plate of the fuel cell stack, the reactant gases (the air including the hydrogen as fuel gas and the oxygen as oxidant gas) and the coolant, which are supplied through the inlet manifold holes 12, 14, and 16, are distributed to the flow fields (cathode/anode/coolant channels) of each cell to react and be cooled. Then, the reactant gases and the coolant are merged in the outlet manifold holes 13, 15, and 17 to be discharged to the outside of the stack, as illustrated in FIG. 2.
FIGS. 3A and 3B are cross-sectional views of the fuel cell illustrating a cathode channel and an anode channel through which reactant gases flow, and a coolant channel. Reference numeral 21 refers to an MEA including a catalyst layer (a catalyst electrode, i.e. a cathode and an anode).
Here, each portion at which the bipolar plate 10 is in contact with the GDL 22 refers to a land section 10a, and each portion at which one bipolar plate is in contact with another bipolar plate refers to a channel section 10b. 
In addition, flow fields formed by the channel section 10b refer to channels through which the reactant gases flow, which are a cathode channel (air channel) 11a through which air (oxygen) flows and an anode channel (hydrogen channel) 11b through which hydrogen flows. A flow field formed by the land section 10a refers to a coolant channel 11c through which coolant flows.
The flow fields of the bipolar plate are classified into the cathode channel 11a, the anode channel 11b, and the coolant channel 11c, and the air, hydrogen, and coolant flow in a parallel direction to the flow fields of the bipolar plate. The bipolar plate manufactured by processing a metal material using a press causes design restrictions due to the shape of the bipolar plate itself.
The metallic bipolar plate has the flow fields designed in various manners because it is difficult to achieve a complex shape, but the flow field pattern has the same form as a typical channel shape.
That is, the flow fields through which the reactant gases flow have relief and intaglio patterns formed on a flat and thin plate metal material, and the coolant or other gases flow through the flow fields formed on the opposite surface thereof.
In addition, the conventional bipolar plate generally has long channels which are arranged in parallel throughout the reaction region or has inclined flow fields. The bipolar plate has advantages and disadvantages in terms of performance, pressure characteristics, and drainage characteristics according to the design of the flow fields of the bipolar plate. However, the flow fields having a rectangular cross section, a trapezoid cross section, or a cross section similar to the same are commonly formed in a portion corresponding to the reaction region of the bipolar plate such that the reactant gases are supplied therethrough.
The bipolar plate has a portion in which flow fields are formed and another portion in which flow fields are not formed. The portion, in which flow fields are formed, is a flow field section (including the above channels) having the flow fields for reactant gases, and the other portion, in which flow fields are not formed, is a land section.
The flow field section is typically distinguished from the land section in the bipolar plate. The diffusion amount of gas transferred to the GDL varies due to a flow difference between the flow field section and the land section. This nonuniformity causes a concentration difference between the flow field section and the land section in the MEA in which the electrochemical reaction occurs. For this reason, it is difficult to expect uniform power generation in the entire reaction region due to a difference in the electrochemical reaction.
In the conventional bipolar plate, the reactant gases such as air and hydrogen flow in the direction perpendicular to the direction in which the substances are transferred to the catalyst layer where the electrochemical reaction occurs. For this reason, the bipolar plate has a disadvantage in that the substances are transferred to the catalyst layer depending on only diffusion by a concentration difference and a partial pressure difference between the channels 11a and 11b and the MEA 21.
That is, since the flow direction of the reactant gases is perpendicular to the direction in which the substances are transferred to the catalyst layer where the electrochemical reaction occurs, the substances are transferred through the GDL 22 to the catalyst layer using only diffusion by a pressure difference at the inlet and the outlet between the flow field channels 11a and 11b for the reactant gas and a concentration difference between the channels 11a and 11b and the catalyst layer.
This method is a passive transfer method in terms of supplying the reactant gases to a required portion. Therefore, it is difficult to transfer the substances to the catalyst layer by the flow in the bipolar plate.
Therefore, the limiting current density of the fuel cell decreases, and thus, the performance of the fuel cell may deteriorate. In addition, the performance of the fuel cell may not be improved in a high power section, and it is difficult to discharge water as a by-product produced by the electrochemical reaction since it is difficult to remove water present in the GDL.
Moreover, since the reactant gas concentration required for the electrochemical reaction is not transferred to the catalyst layer in the rear end portion (the outlet portion) of the flow field channel, a power loss may result.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention, and therefore, it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.